1. Field of Invention
This invention relates to an apparatus and method for testing sheet material such as bank notes or papers of value.
2. Description of Related Art
EP-A 0 537 513 shows such an apparatus wherein the authenticity of bank notes is tested. For this purpose the bank note is first illuminated with light-emitting diodes in the colors, red and green, and in the infrared spectral region. The light-emitting diodes are pulsed sequentially so that the bank note is only illuminated with one color or in the infrared spectral region at a time. The diodes are disposed on both sides of the bank note. The light transmitted or reflected by the bank note is detected by means of a single linear CCD array and converted into electric signals which are then further processed accordingly.
By using a linear CCD array one can achieve a relatively high local resolution on the bank note. However, a disadvantage is the high equipment expense necessary for illuminating the bank note in different colors or in the infrared spectral region. A special problem is mutually balancing the illumination levels in the different colors or in the infrared spectral region.
DE-OS 38 15 375 shows a further apparatus for testing the authenticity of sheet material. It is constructed from a plurality of like modules. Each module has its own illumination device with white light and a linear photodiode array. Further, before each module is a filter which transmits for example the colors, red, green or blue, or in the infrared spectral region.
For testing the sheet material, the light emitted by the illumination device passes through the filter of the corresponding module and illuminates the sheet material. The light diffusely reflected by the sheet material then passes through the filter of the module and is detected by the linear photodiode array. The latter converts the impinging light into corresponding signals which are then further processed.